10 Facts About Planet Mercury

Messenger Spacecraft Images of Mercury - Mercury -- In Color!!
A color image of Mercury sent back by the MESSENGER mission. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The planets of the solar system are a fascinating collection of different types of worlds. The rocky planets orbit close to the Sun, while the gas and ice giants are farther away. The rocky planet Mercury is a fascinating place, still full of surprises for the scientists who study it.

Edited by Carolyn Collins Petersen.

01
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Mercury Is the Closest Planet to the Sun

Messenger Spacecraft Images of Mercury - Messenger Views Mercury's Horizon
Mercury's rugged surface tells a story of impacts and volcanism. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury speeds around the Sun at 50 kilometers per second. That's faster than any other planet. Its path is a wildly elliptical (non-circular) orbit. It comes as close as 47 million kilometers and as far as 70 million kilometers from the Sun. Mercury's orbit is 88 Earth-days long. 

The surface of Mercury is heavily cratered in some areas ​and sports giant cracks that formed as the planet cooled. 

02
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Mercury's Existence Has Been Known Since Antiquity

Mercury - Visual Solar System Tour
Mercury, the planet closest to the Sun. USGS

The Greeks gave Mercury two names: Apollo, for when it appeared as a morning star, and Hermes, for when it showed up in the sky as an evening star. It wasn't until astronomers looked at Mercury through telescopes that they proved that Mercury was a planet and not a star.

03
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Mercury's Surface Closely Resembles Earth's Moon

Messenger Spacecraft Images of Mercury - MESSENGER Views an Intriguing Crater
Messenger Spacecraft Images of Mercury - MESSENGER Views an Intriguing Crater. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury is scarred by thousands of impact craters resulting from collisions with meteors. While there are areas of smooth terrain, there are also cliffs, some soaring up to a mile high, formed by ancient impacts. Its surface color is very dark, likely due to cometary impacts scattering ice on the surface, which then dissipated, leaving behind a coating of carbon "dust". 

Other cracks on Mercury's surface formed early in its history as the molten surface cooled and split apart while Mercury shrank. 

04
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Mercury Has a Large, Partially-molten Iron Core

Mercury Image - 10 Things You Should Know About Mercury
Mercury Image. NASA

Mercury's interior is made of a large iron core that is partially molten. It has a radius of 1,800 to 1,900 km, which is almost 75 percent of the planet's diameter and nearly the size of Earth's Moon. Mercury's outer shell, comparable to Earth's outer shell (called the mantle) is only 500 to 600 km thick.

05
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Mercury Has a Very Thin, Almost Non-existent Atmosphere

Messenger Spacecraft Images of Mercury - A Closer Look at the Previously Unseen Side
Messenger Spacecraft Images of Mercury - A Closer Look at the Previously Unseen Side. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury's atmosphere is incredibly thin and tenuous. What little "air" there is comes from atoms blasted off its surface and particles from the solar wind. These atoms quickly escape into space. Unlike the Earth and Venus, which have stable atmospheres, Mercury's tenuous atmospheric envelope of is constantly being replenished and lost.

06
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Mercury Is a Planet of Extremes

Messenger Spacecraft Images of Mercury - Counting Mercury's Craters
Messenger Spacecraft Images of Mercury - Counting Mercury's Craters. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Temperatures on Mercury swing from a low of 90 K to more than 700 K. Venus actually experiences hotter temperatures at the high end of the scale. There are parts of Mercury that never see sunlight. These places (at the poles) also contain water ice that never melts. That ice likely was scattered there from comet impacts. 

07
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Mercury Has No Moons

Mercury Pictures Gallery - Mercury's South Pole
This is Mercury's south polar region, photographed by the Mariner spacecraft as it flew by. NASA/JPL/Northwestern University

While Mercury may resemble our own Moon in many ways, it has no moon of its own. It may never have had one, or if it did, likely lost it during the very earliest part of solar system history. 

08
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Astronomers Once Thought the Same Side of Mercury Faced the Sun

Mercury - Visual Solar System Tour
Mercury - Visual Solar System Tour - Mercury. USGS

In 1965, astronomers discovered that Mercury completes three rotations for every two orbits around the Sun. If you wanted to stay up for a Mercury day, you would go through the equivalent of 176 Earth days.

09
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Mercury's Caloris Basin Is Large, Rougly 1,300 Kilometers Across

MESSENGER Settles Old Debates and Makes New Discoveries at Mercury
MESSENGER Settles Old Debates and Makes New Discoveries at Mercury. NASA/JPL

Like other planets, Mercury has some distinctive surface features. One of its most striking is the Caloris Basin, the largest on the planet. This wide plain formed as a result of an asteroid impact on the planet's surface early in the solar system's history. Over the next half-billion years, the planet actually shrank as Mercury cooled from its formation. The outer crust, called the lithosphere, was compressed and grew strong enough to prevent the planet's magma from reaching the surface. That effectively ended the planet's period of geologic activity.

10
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Mercury Is the Least-explored of Our Solar System's Inner Planets

Mariner 10 Spacecraft
Mariner 10 Spacecraft. NASA

Only two spacecraft have visited Mercury: Mariner 10 in 1974-75. Mariner 10's discovery that Mercury has a very weak magnetic field, similar to but weaker than Earth's, was a major surprise. The MESSENGER mission (short for Mercury Surface, Space Environment, Geochemistry, and Ranging) orbited the planet from 2011 until 2015, when it plunged into the planet's surface. The spacecraft's images and data gave planetary scientists close-up looks at the surface and helped them understand Mercury's interior. 

The next mission to Mercury is called BepiColumbo ​and will arrive at the planet in 2025. The plans call for it to orbit Mercury for at least a year and carry out further observations of this closest planet to the Sun. 

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Greene, Nick. "10 Facts About Planet Mercury." ThoughtCo, Nov. 9, 2017, thoughtco.com/things-you-should-know-about-mercury-3073448. Greene, Nick. (2017, November 9). 10 Facts About Planet Mercury. Retrieved from https://www.thoughtco.com/things-you-should-know-about-mercury-3073448 Greene, Nick. "10 Facts About Planet Mercury." ThoughtCo. https://www.thoughtco.com/things-you-should-know-about-mercury-3073448 (accessed November 24, 2017).